ON ARTIFICIAL MODIFICATION OF LIGHT 



REACTIONS AND THE INFLUENCE OF 



ELECTROLYTES ON PHOTOTAXIS 



WOLFGANG F. EWALD 



Since Loeb's early papers on animal heliotropism it has been 

 known that in certain cases the experimenter is able to bring 

 about changes in the orientation of animals to light by artificial 

 means. Loeb was able to show that in marine copepods decrease 

 of temperature and increase of concentration of the seawater 

 made negatively phototactic animals positive, and vice versa: 

 that increase of temperature and decrease of concentration made 

 positive animals negative. Similar effects of temperature were 

 noted in Polygordius larvae. Strassburger ('78), Massart, Holmes 

 and Mast found the opposite effects in swarm-spores, Protozoa 

 and a species of Ranatra. Later on Loeb supplemented his 

 communications by the discovery, that freshwater Gammarus, 

 Daphnia and Volvox could be made strongly positive by adding 

 acids, especially CO2, to the water. The same clear results 

 could not, however, be obtained in marine planktonic forms. 

 In the nauplius of Balanus perforatus Loeb could observe a 

 strong influence on phototaxis only by exposing the animals to 

 very strong or very weak light. Strong light made positive 

 animals negative, weak light made negative animals positive. 

 The ultra-violet rays proved to be specially effective in making 

 positive animals negative. In fact, their effect was stronger 

 than that of all othel- rays combined. Rothert, in experiment- 

 ing on the effect of alcohol, ether and chloroform on free swim- 

 ming plants, found the light reactions of Gonium and Pandorina 

 to be inhibited by some of these narcotics in a certain dosation, 

 the power of locomotion remaining unaffected. 



According to the investigations of Tappeiner and Hertel a 

 similar modification of light reactions is brought about by cer- 



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